Add introductory statement to migration guides section (#8152)

Addresses feedback on #8141 to clarify migration guide selection for
users.

## Changes

- Added introductory statement "Select the guide that matches your
current MSAL version:" before the migration guides list in
`lib/msal-browser/README.md`

The statement precedes the reverse-chronological list of migration
guides (v4→v5, v3→v4, v2→v3, v1→v2) to help users quickly identify which
guide applies to their current version.

<!-- START COPILOT CODING AGENT TIPS -->
---

💬 We'd love your input! Share your thoughts on Copilot coding agent in
our [2 minute survey](https://gh.io/copilot-coding-agent-survey).

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: hectormmg <5218163+hectormmg@users.noreply.github.com>
Co-authored-by: Hector Morales <hemoral@microsoft.com>

Author: Copilot

lib/msal-browser/README.md

@@ -69,6 +69,8 @@ yarn add @azure/msal-browser
 
 ### Migrating from Previous MSAL Versions
 
+Select the guide that matches your current MSAL version:
+
 -   [Migrating from MSAL v4.x to MSAL v5.x](https://github.com/AzureAD/microsoft-authentication-library-for-js/blob/dev/lib/msal-browser/docs/v4-migration.md)
 -   [Migrating from MSAL v3.x to MSAL v4.x](https://github.com/AzureAD/microsoft-authentication-library-for-js/blob/dev/lib/msal-browser/docs/v3-migration.md)
 -   [Migrating from MSAL v2.x to MSAL v3.x](https://github.com/AzureAD/microsoft-authentication-library-for-js/blob/dev/lib/msal-browser/docs/v2-migration.md)